Solution for making photoconductive layers in CRTS

ABSTRACT

Disclosed is a solution for forming photoconductive layer for electrophotographically manufacturing a luminescent screen on an interior surface of a faceplate panel for a CRT. The solution consists of 0.01 to 10% by weight of bis dimethyl phenyl diphenyl butatriene, 1 to 30% by weight of polystyrene, 30 to 100% by weight of 2,5-bis(4-diethyl aminophenyl)-1,3,4-oxadiazole, against 100% by weight of polystyrene and the remainder of solvent. The photoconductive layer manufactured using the solution facilitates performing all the processes in a visible light environment with safety in work operations, because the solution contains 2,5-bis(4-diethyl aminophenyl)-1,3,4-oxadiazole, instead of trinitro fluorenone(TNF) of cancer-causing material, as ultraviolet-sensitive material.

FIELD OF THE INVENTION

The present invention relates to a solution for making a photoconductivelayer and a method of electrophotographically manufacturing a viewingscreen for a cathode ray tube(CRT) using the solution, and moreparticularly to a photoconductive solution which does not cause cancerand facilitates performing all the processes under a visible lightenvironment with safety in work operations.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, a color CRT 10 generally comprises an evacuatedglass envelope consisting of a panel 12, a funnel 13 sealed to the panel12 and a tubular neck 14 connected by the funnel 13, an electron gun 11centrally mounted within the neck 14 and a shadow mask 16 removablymounted to a sidewall of the panel 12. A three color phosphor screen isformed on the inner surface of a display window or faceplate 18 of thepanel 12.

The electron gun 11 generates three electron beams 19a or 19b, saidbeams being directed along convergent paths through the shadow mask 16to the screen 20 by means of several lenses of the gun and a highpositive voltage applied through an anode button 15 and being deflectedby a deflection yoke 17 so as to scan over the screen 20 throughapertures or slits 16a formed in the shadow mask 16.

In the color CRT 10, the phosphor screen 20, as shown in FIG. 2,comprises an array of three phosphor elements R, G and B of threedifferent emission colors arranged in a cyclic order of a predeterminedstructure of multiple-stripe or multiple-dot shape and a matrix oflight-absorptive material surrounding the phosphor elements R, G and B.

A thin film of aluminum 22 overlies the screen 20 in order to provide ameans for applying the uniform potential applied through the anodebutton 15 to the screen 20, increase the brightness of the phosphorscreen and prevent from degrading ions in the phosphor screen anddecreasing the potential of the phosphor screen. And also, a film ofresin such as lacquer(not shown) may be applied between the aluminumthin film 22 and the phosphor screen to enhance the flatness andreflectivity of the aluminum thin film 22.

In a photolithographic wet process, which is well known as a prior artprocess for forming the phosphor screen, a slurry of a photosensitivebinder and phosphor particles is coated on the inner surface of thefaceplate. It does not meet the higher resolution demands and requires alot of complicated processing steps and a lot of manufacturingequipments, thereby necessitating a high cost in manufacturing thephosphor screen. And also, it discharges a large quantity of effluentsuch as waste water, phosphor elements, 6th chrome sensitizer, etc.,with the use of a large quantity of clean water.

To solve or alleviate the above problems, the improved process ofelectrophotographically manufacturing the screen utilizing dry-powderedphosphor particles is developed. U.S. Pat. No. 4,921,767, issued toDatta at al. on May 1, 1990, describes one method ofelectrophotographically manufacturing the phosphor screen assembly usingdry-powdered phosphor particles through the repetition of a series ofsteps represented in FIGS. 3A to 3E, as is briefly explained in thefollowing.

Prior to the electrophotographic screening process, foreign substance isclearly removed from an inner surface of a panel by several conventionalmethods. Then, a conductive layer 32, as shown in FIG. 3A, is formed byconventionally coating the inner surface of the viewing faceplate 18with a suitable conductive solution comprising an electricallyconductive material which provides an electrode for an overlyingphotoconductive layer 34. The conductive layer 32 can be an inorganicconductive material such as tin oxide or indium oxide, or their mixtureor, preferably, a volatilizable organic conductive material consistingof a polyelectrolyte commercially known as polybrene(1,5-dimethyl-1,5-diazaundecamethylene polymethobromide, hexadimethrinebromide), available from Aldrich Chemical Co., Milwaukee Wis., oranother quaternary ammonium salt. The polybrene is conventionallyapplied to the inner surface of the viewing faceplate 18 in an aqueoussolution containing about 10 percent by weight of propanol and about 10percent by weight of a water soluble, adhesion promoting polymer such aspoly(vinyl alcohol), polyacrylic acid, certain polyamide and the like,and the coated solution is dried to form the conductive layer 32 havinga thickness from about 1 to 2 microns and a surface resistivity of lessthan about 10₈ ohms per square unit.

The photoconductive layer 34 is formed by coating the conductive layer32 with a photoconductive solution comprising a volatilizable organicpolymeric material, a suitable photoconductive dye and a solvent. Thepolymeric material is an organic polymer such as polyvinyl carbazole, oran organic monomer such as n-ethyl carbazole, n-vinyl carbazole ortetraphenylbutatriene dissolved in a polymeric binder such aspolymethylmethacrylate or polypropylene carbonate. The suitable dyes,which are sensitive to light in the visible spectrum, preferably fromabout 400 to 700 nm, include crystal violet, chloridine blue, rhodamineEG and the like. This dye is typically present in the photoconductivecomposition in from about 0.1 to 0.4% by weight. The solvent for thephotoconductive composition is an organic such as chlorobenzene orcyclopentanone and the like which will produce as little crosscontamination as possible between the layers 32 and 34. Thephotoconductive solution is conventionally applied to the conductivelayer 32, as by spin coating, and dried to form a layer having athickness from about 2 to 6 microns.

FIG. 3B schematically illustrates a charging step, wherein thephotoconductive layer 34 overlying the conductive layer 32 is positivelycharged in a dark environment by a conventional positive coronadischarger 36, which moves across the layer 34 and charges it within therange of +200 to +700 volts.

FIG. 3C schematically shows an exposure step, wherein the shadow mask 16is inserted in the panel 12 and the charged photoconductor is exposedthrough a lens system 40 and the shadow mask 16, to the light from axenon flash lamp 38 disposed at one position within a conventionalthree-in-one lighthouse. Then, the positive charges of the exposed areasare discharged through the grounded conductive layer 132 and the chargesof the unexposed areas remain in the photoconductive layer 134, thusestablishing a latent charge image in a predetermined array structure.Three exposures are required for forming a light-absorptive matrix withthree different incident angles, respectively.

FIG. 3D schematically represents a developing step, wherein the shadowmask 16 is removed from the panel 12 and the positively or negativelycharged, dry-powdered particles are expelled from the developer anddeposited to one of the charged, unexposed areas and the discharged,exposed areas depending on the polarity of the charged particles due toelectrical attraction or repulsion, thus one of the two areas isdeveloped in a predetermined array pattern. The deposited particles arefixed to the photoconductive layer 34 as described hereinafter. Thelight-absorptive material particles for directly developing theunexposed or positively charged areas are charged negatively and thephosphor particles are positively charged for reversely developing theexposed or discharged areas. The charging of the dry-powdered particlesis executed by a triboelectrical charging method using surface-treatedcarrier beads.

The dry-powdered particles and the surface-treated carrier beads, coatedwith a thin film of a suitable charge-control agent, are mixed in thedeveloper 42. The black matrix particles or phosphor particles arenegatively or positively charged by the surface-treated carrier beadsdepending upon the suitable charge-control agent.

FIG. 3E schematically represents a fixing step, wherein infraredradiation is used to fix the deposited particles by melting or thermallybonding the polymer component of the particles 21 to the photoconductivelayer 34. Accordingly, polymers to be thermally bonded are contained inthe photoconductive layer 34 and the black matrix particles or phosphorparticles.

The steps of charging, exposing, developing and fixing are repeated forthe black matrix particles and the three different phosphor particles.The faceplate panel 12 is baked in air at a temperature of 425 degreescentigrade, for about 30 minutes to drive off the volatilizableconstituents of screen including the conductive layer 32, thephotoconductive layer 34, the solvents present in both the screenstructure materials and in the filming lacquer, thereby forming anscreen array of light-absorptive material 21 and three phosphor elementsR, G and B in FIG. 2.

The aforementioned process has one problem that it requires darkenvironment during performing all the steps since the photoconductivelayer is sensitive to the visual light.

Also, U.S. Pat. No. 5,413,885 discloses a method ofelectrophotographically manufacturing the CRT screen under low intensityyellow lights of 577-597 nm using a novel photoconductive layer to solvethe aforementioned problem. The photoconductive layer comprisesultraviolet-sensitive material consisting of bis dimethyl phenyldiphenyl butatriene, and one of trinitro fluorenone(TNF),ethylanthraquinone(EAQ) and their mixture.

However, the ultraviolet-sensitive material TNF is cancer-causingmaterial and affects badly the human body.

Accordingly, it is an object of the present invention to provide asolution for making a photoconductive layer in a method ofelectrophotographically manufacturing a viewing screen for a cathode-raytube(CRT), which does not cause cancer simultaneously with requiring nodark environment and facilitates performing all the processes in avisible light environment with safety in work operations.

SUMMARY OF THE INVENTION

To overcome the foregoing problem, the present invention is to provide asolution for forming photoconductive layer for electrophotographicallymanufacturing a luminescent screen on an interior surface of a faceplatepanel for a CRT, said solution containing 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, instead of trinitrofluorenone(TNF) ofcancer-causing material, as ultraviolet-sensitive material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view partially in axial section of a color cathode-raytube.

FIG. 2 is an enlarged section of a screen assembly of the tube shown inFIG. 1.

FIGS. 3A through 3E show various steps in electrophotographicallymanufacturing the screen assembly of the tube by viewing a portion of afaceplate having a conductive layer and an overlying photoconductivelayer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is to provide a solution for formingphotoconductive layer for electrophotographically manufacturing aluminescent screen on an interior surface of a faceplate panel for a CRTcomprising the steps of coating said surface of the panel with avolatilizable conductive layer and an overlying volatilizablephotoconductive layer, establishing a substantially uniformelectrostatic charge over the whole area of the inner surface of saidphotoconductive layer, exposing selected areas of said photoconductivelayer to discharge the charge from the selected areas, developing one ofthe charged, unexposed areas and the discharged, exposed areas dependingupon the polarity of the charged particles with one of charged phosphorparticles and light-absorptive material particles, said solutionconsisting of 0.01 to 10% by weight of bis dimethyl phenyl diphenylbutatriene, 1 to 30% by weight of polystyrene, 30 to 100% by weight of2,5-bis(4-diethyl aminophenyl)-1,3,4-oxadiazole, against 100% by weightof polystyrene and the remainder of solvent.

It is desirable that said 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole be 70% by weight against 100% by weight ofpolystyrene.

In the solution of the present invention, 0.01 to 10% by weight of bisdimethyl phenyl diphenyl butatriene is used as one ofultraviolet-sensitive material. It is not desirable to contain said bisdimethyl phenyl diphenyl butatriene below 0.01 wt. % or above 10 wt. %because in the case of containing below 0.01 wt. % the photoconductivelayer does not act as the ultraviolet-sensitive layer and in the case ofover 10 wt. % foreign substance comes into being existence and iscoagulated or bubble is generated on the photoconductive layer.

Said polystyrene is used as polymer binder.

In the solution of the present invention, 30 to 100% by weight of2,5-bis(4-diethyl aminophenyl)-1,3,4-oxadiazole against 100% by weightof polystyrene is used as another ultraviolet-sensitive material, whichsubstitutes for the forgoing trinitro fluorenone(TNF) of cancer-causingmaterial for safe working. It is also undesirable to contain said2,5-bis(4-diethyl aminophenyl)-1,3,4-oxadiazole below 30 wt. % or above100 wt. % against 100% by weight of polystyrene because in the case ofcontaining below 30 wt. %, the photoconductive layer does not act as theultraviolet-sensitive layer and in the case of over 100 wt. % foreignsubstance comes into existence and is coagulated or bubble is generatedon the photoconductive layer.

Said solvent for dissolving the polymer binder is selected from benzene,benzene derivatives or their mixture, etc., said benzene derivativesincluding toluene, ethylbenzene, xylene, styrene, etc.

The aforementioned solution of the present invention is used inelectrophotographically manufacturing a luminescent screen on aninterior surface of a faceplate panel for a CRT as in the following.

In FIG. 3A the inner surface of a panel 18 is coated with avolatilizable conductive layer 32 as described in the forgoing prior artand then with an overlying volatilizable photoconductive layer 34 usingthe forgoing solution of the present invention.

The photoconductive layer 34 is charged with positive electrostaticcharge over the whole area of the inner surface thereof and then, saidphotoconductive layer is exposed in selected areas thereof to dischargethe charge from the selected areas, developing one of the charged,unexposed areas and the discharged. The exposed areas are developed withcharged phosphor particles and said developed phosphor particles arefixed on the photoconductive layer 34, such steps being performed underthe visual light. And then the screen is formed as described in theforgoing prior art.

The aforementioned solution of the present invention, removes danger ofcancer since it contains 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, instead of trinitro-fluorenone(TNF) ofcancer-causing material, and does require dark environment because thephotoconductive materials are sensitive to ultraviolet rays only,thereby ensuring worker's safety and improvement of the workingcondition.

It should be clear to one skilled in the art that the present solutioncan be used for electrophotographically manufacturing the screen by themethod as described in U.S. Pat. No. 4,921,767 and that the presentprocess for obtaining the present solution can be modified within thescope of the present invention.

What is claimed is:
 1. A solution for forming a photoconductive layerfor electrophotographically manufacturing a luminescent screen on aninterior surface of a faceplate panel for a CRT of a type created bycoating the interior surface of the faceplate panel with a volatilizableconductive layer and an overlying volatilizable photoconductive layer,establishing a substantially uniform electrostatic charge over the wholearea of the inner surface of the photoconductive layer, exposingselected areas of the of the photoconductive layer to discharge thecharge from the selected areas, developing one of the charged, unexposedareas and the discharged, exposed areas with one of charged phosphorparticles and light-absorptive material particles, depending upon thepolarity of the charged particles, the solution consisting of:0.01 to10% by weight of bis dimethyl phenyl diphenyl butatriene; 1 to 30% byweight of polystyrene; 30 to 100% by weight of 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole based on 100% by weight of polystyrene;and the remainder being a solvent.
 2. The solution according to claim 1,wherein the 2,5-bis(4-diethyl aminophenyl)-1,3,4-oxydiazole is 70% byweight based on 100% by weight of polystyrene.
 3. A method ofelectrophotographically manufacturing a luminescent screen on aninterior surface of a faceplate panel for a CRT, the methodcomprising:coating the interior surface of the faceplate panel with avolatilizable conductive layer and an overlying volatilizablephotoconductive layer, the overlying volatilizable photoconductive layerbeing formed by applying and then drying a solution having 0.01 to 10%by weight of bis dimethyl phenyl diphenyl butatriene, 1 to 30% by weightof polystyrene, 30 to 100% by weight of 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole based on 100% by weight of polystyrene,and the remainder being a solvent; establishing a substantially uniformelectrostatic charge over the whole area of the inner surface of thephotoconductive layer; exposing selected areas of the of thephotoconductive layer to discharge the charge from the selected areas;and developing one of the charged, unexposed areas and the discharged,exposed areas with one of charged phosphor particles andlight-absorptive material particles, depending upon the polarity of thecharged particles.